ARIA: brain swelling and bleeding with anti-amyloid drugs

ARIA is the main side effect of the newer Alzheimer's drugs that remove amyloid (a sticky protein that builds up in the brain). The two drugs in use in the U.S. are lecanemab (Leqembi) and donanemab (Kisunla). ARIA means brain swelling or small bleeds that show up on an MRI scan.^1,2

Most people with ARIA never feel it. But it can be serious, and in rare cases it has caused death. Knowing the warning signs and keeping every MRI appointment are the best ways to stay safe. This page explains what families need to know.

What ARIA is

ARIA stands for amyloid-related imaging abnormalities. "Imaging" means it is seen on a brain scan. Doctors split it into two types:^1,2

• ARIA-E (edema): swelling, or a buildup of fluid, in part of the brain or in the grooves on its surface.
• ARIA-H (hemosiderin): very small spots of bleeding called microbleeds, or a thin coating of old blood on the brain's surface (superficial siderosis).

The two types often happen together.¹ Scientists are still studying exactly why ARIA happens.⁸ It seems to be linked to amyloid that builds up in the walls of small blood vessels in the brain.

Amyloid in blood vessel walls is called cerebral amyloid angiopathy (CAA). People who already have signs of CAA on their MRI, such as several old microbleeds, are more likely to get ARIA.^1,4

How common is ARIA?

The numbers below come from the FDA labels, which use the large trials that led to approval. Some people on placebo (a look-alike with no medicine) also showed these changes, so not every case is caused by the drug.^1,2

Sources for the table: FDA labels for Leqembi and Kisunla.^1,2 Lecanemab results are from an 18-month trial. Donanemab results are from a 1-year study of the newer, slower starting schedule. The placebo range comes from both drugs' main trials. The trials were different, so do not compare the two drugs directly from this table.

Donanemab's newer schedule lowered the risk. In July 2025, the FDA approved a new way to start donanemab. The dose goes up more slowly over the first few months. In a study, swelling (ARIA-E) at one year dropped from about 25% with the old schedule to about 16% with the new one.² On the original schedule, about 6 in 100 people had swelling with symptoms.²

A large Cochrane review in April 2026 pooled 11 trials of drugs in this class, Brain swelling happened in about 12 of every 100 people who got an anti-amyloid drug, compared with about 1 in 100 on placebo.⁶

When ARIA happens

ARIA is most likely early in treatment.^1,2

• Lecanemab: most swelling shows up within the first 7 doses. The label asks for extra watchfulness during the first 14 weeks.¹
• Donanemab: most swelling shows up within the first 24 weeks (about 6 months).²

ARIA can still happen later, and some people have it more than once.¹

In August 2025, the FDA reviewed reports of serious swelling in people taking lecanemab. It found six deaths. Five of the six people had symptoms within about a week of a recent infusion, after the 3rd or 4th infusion.³ This is why the FDA added an earlier MRI to the lecanemab schedule (see below).³

ARIA usually goes away. On lecanemab, swelling had cleared on MRI in about half of people by 12 weeks and in about 8 in 10 by 17 weeks.¹ Donanemab showed a similar pattern.² Symptoms went away in most people who had them.^1,2 Small bleeds usually stay visible on the scan. Doctors check that they are stable before going on.¹

Symptoms to watch for

Many people with ARIA have no symptoms at all. It is found only on a routine scan.^1,3 When symptoms do occur, they can include:^1,3

• A new or worse headache
• More confusion than usual
• Dizziness or feeling off balance
• Vision changes
• Nausea
• Trouble walking
• Trouble finding words or speaking
• Weakness
• Seizures

Some of these signs look like Alzheimer's getting worse, such as more confusion.⁵ A care partner who sees the person every day is often the first to notice. Write down what you see and when it started. A memory notebook can help.

Before starting treatment, talk with your doctor about how a stroke would be handled.⁴ Many families keep a card in the wallet and a note on the phone that names the drug and the prescribing clinic's phone number. See our page on hospital stays and emergency rooms.

The APOE e4 gene and ARIA risk

APOE is a gene everyone has. One form, called e4, raises the risk of Alzheimer's. It also raises the risk of ARIA.^1,2 You get one copy of APOE from each parent, so a person can have zero, one or two copies of e4.

Sources for the table: FDA labels for Leqembi and Kisunla.^1,2

People with two copies also have more serious ARIA. On lecanemab, about 9 in 100 people with two copies had swelling with symptoms, compared with about 1 in 100 people with no copies.¹ With donanemab's newer starting schedule, swelling in people with two copies dropped from about 57% to about 24%. That group was small, so the result is less certain.²

The FDA labels recommend an APOE test before starting either drug.^1,2 Your doctor should explain what the result could mean before you test.⁷ A result also says something about your children and siblings, since they may share the gene.⁵ Read more on genes and dementia.

Blood thinners: an important caution

Many older adults take a blood thinner for atrial fibrillation (an irregular heartbeat), a past clot or a heart valve. These medicines matter here because ARIA involves bleeding.

• Anticoagulants (such as warfarin, apixaban, rivaroxaban, dabigatran or heparin): In the lecanemab trial, 2.5% of people who took one of these had a brain bleed larger than 1 cm, compared with none on placebo.¹ Expert advice says people who need these medicines should not start lecanemab. If someone on the drug later needs an anticoagulant, infusions should be paused.⁵
• Aspirin and similar platelet drugs (such as clopidogrel): These were allowed in the trials and do not by themselves rule a person out.⁵ Overall, they did not raise the rate of ARIA.¹ For donanemab, most of the data come from aspirin, so less is known about other drugs.²
• Clot-busting drugs for stroke (thrombolytics): See the warning box above. The American Heart Association advises that doctors and patients talk about stroke treatment risks before anti-amyloid treatment begins.⁴

Other reasons for caution

The FDA labels and expert advice point to these concerns:^1,4,5

• An MRI before treatment showing two or more old microbleeds, any area of superficial siderosis, or a larger past brain bleed. These suggest CAA and raise the risk of ARIA.
• A lot of damage to small blood vessels on the MRI, or past strokes.
• A history of seizures, since severe ARIA can cause seizures.

Lecanemab's main trial left out people with more than four microbleeds or a past bleed larger than 1 cm.¹ Doctors often advise against treatment for these people. Our page Are anti-amyloid treatments right for us? explains who may not be a good fit.

The MRI schedule

MRI scans catch ARIA early, usually before it causes symptoms. Each drug has its own schedule.^1,2,3

For lecanemab, the scan before the 3rd infusion was added in August 2025 after the FDA's safety review.³ For both drugs, the MRI should be read before the next dose is given.¹ With either drug, ask for an MRI right away if new symptoms appear.

Tips for MRI days:

• Put every scan on a shared calendar as soon as it is booked.
• Ask who reads the scan and how fast you will hear the result.
• If the person gets anxious in the scanner, ask the team about ways to help.

What happens if ARIA shows up

What the doctor does depends on how large the ARIA looks on the MRI and whether there are symptoms.^1,2

• Mild ARIA with no symptoms: treatment may go on, with close MRI follow-up.
• Moderate or severe ARIA, or any symptoms: doses are usually paused. They restart only after the MRI shows the swelling has cleared or the bleeding has settled, and symptoms are gone.
• Severe bleeding or a large bleed: the doctor may stop the drug for good.

A follow-up MRI is often done 2 to 4 months after ARIA is found.¹ Serious ARIA may need a hospital stay. Some experts use high-dose steroids for severe ARIA, but more research is needed.^4,5 Seizures from ARIA need prompt treatment.⁵

The choice to restart after ARIA should be made together with the person and family.⁵ Expert groups keep updating their advice. In April 2026, an Alzheimer's Association workgroup published new recommendations on ARIA in everyday care, including blood thinners and stroke treatment.⁸

Questions to ask the doctor

Before starting treatment

• What is my personal risk of ARIA, based on my MRI and genes?
• Should I have an APOE test? What will the result change?
• I take aspirin, a blood thinner or supplements. Is that safe with this drug?
• How would a stroke be handled if I am on this drug?

During treatment

• When are my MRI scans, and who tells me the results?
• Which symptoms mean I should call you, and which mean 911?
• Who do we call at night or on a weekend?
• Can I have a wallet card that names the drug?

If ARIA is found

• Is it ARIA-E, ARIA-H, or both? How large is it?
• Should we pause treatment? When will the next MRI be?
• What are the chances it comes back if we restart?

For help finding doctors or understanding treatment, the Alzheimer's Association 24/7 Helpline is 800-272-3900. See also dementia medicines and paying for new Alzheimer's drugs.

Education only. This page is general information written from the sources listed. It is not medical, legal or financial advice and does not replace a doctor, therapist or lawyer who knows your situation. How we write and check pages.